Earliest siphuncle-bearing cephalopod from the early Cambrian

Recent cephalopods are the most intelligent invertebrates and are unique
among molluscs in ancestrally possessing chambered, gas-filled shells
penetrated lengthwise by a siphuncle that regulates buoyancy that,
combined with jet propulsion, enables a nektonic and mostly predatory
lifestyle through jet propulsion. The origin of this key innovation
remains enigmatic, as major gaps exist between the earliest accepted
cephalopod, Plectronoceras cambria from the late Cambrian, and
molecular clock estimates placing the lineage’s divergence from other
molluscs in the early Cambrian. Here we report Eosceras shaanxiense gen.
et sp. nov., a millimetre-sized cephalopod with an orthoconic shell from
the Shuijingtuo Formation (Cambrian Stage 3) of South China. Internally,
the shell exhibits multiple septa and a peripherally situated segmented
tube that appears to bridge the septa through minute canals, indicating
that the soft body migrated anteriorly to form successive chambers during
growth while maintaining contact with earlier chambers via the tube.
Collectively, these features identify the segmented tube in Eosceras as a
candidate primordial cephalopod siphuncle, extending the known range of
stem cephalopods back to the Early Cambrian and also revealing the early
stages in the evolutionary assembly of a chambered phragmocone employed in
buoyancy regulation.